feat(skills): files is the default delivery arm

The A/B has its answer. Across four matched production runs — same recipe,
same task, same three offered skills — the MCP-door arm retrieved 1 in 9 and
the file arm retrieved 3 of 3, with the judge loop closing on the same run
(01a098dd). A signal, not a rate; but 0, 1, 0 → 3 on an otherwise identical
task is not noise, and the mechanism is explained rather than guessed: the
door is a deferred tool the agents never load, and Read is not.

A code default and not CLAWMATES_SKILL_DELIVERY on one server, for the reason
always_inject moved into the skill files: a setting that exists only in one
deployment is a setting nobody can find. The env var still overrides, and
`index` and `inline` stay selectable per mission so the comparison remains
runnable against one binary.

Garbage in the env var still falls to `inline`, not to the default — an
unreadable value must not silently select an arm that needs something
installed. A test pins the default so the next change to it is a decision
made with the numbers in front of you, not a slip.

Co-Authored-By: Claude Opus 5 <[email protected]>
Claude-Session: https://claude.ai/code/session_01WZb5A2kfVfjpdwSochkuHz

crates/cm-api/src/skill_delivery.rs

@@ -46,9 +46,25 @@
 /// straight to `ReadMcpResourceTool`.
 pub const MCP_SERVER: &str = "clawmates_skills";
 
-/// Selects the arm. Unset or unrecognised means [`Mode::Inline`].
+/// Selects the arm. Unset means [`DEFAULT`]; unrecognised means [`Mode::Inline`].
 pub const ENV_VAR: &str = "CLAWMATES_SKILL_DELIVERY";
 
+/// The arm a deployment runs when nothing selects one.
+///
+/// `Files` since 2026-09-13. It was `Inline` — the control arm of an A/B has
+/// to be the thing already running — until the A/B produced its answer: the
+/// MCP-door arm retrieved 1 skill in 9 across three matched production runs,
+/// and the file arm retrieved 3 of 3 on the fourth (`01a098dd`), with the
+/// judge loop closing on the same run. That is a signal and not a rate, but
+/// 0, 1, 0 → 3 on an otherwise identical task is not noise, and a default that
+/// hands agents procedures they demonstrably read beats one that hands them
+/// bodies they were never asked to look for.
+///
+/// A code default and not an env var on one server, because a setting that
+/// exists only in one deployment is a setting nobody can find — the exact
+/// shape `always_inject` had before it moved into the skill files.
+pub const DEFAULT: Mode = Mode::Files;
+
 /// The `# Your skills` preamble under [`Mode::Inline`].
 ///
 /// **Byte-identical to what production has always sent.** The A arm of an A/B
@@ -187,13 +203,15 @@ pub fn parse(s: &str) -> Option<Mode> {
 /// The arm this deployment asks for, before the door is taken into account.
 pub fn requested() -> Mode {
     let Ok(raw) = std::env::var(ENV_VAR) else {
-        return Mode::Inline;
+        return DEFAULT;
     };
     if raw.trim().is_empty() {
-        return Mode::Inline;
+        return DEFAULT;
     }
     match parse(&raw) {
         Some(m) => m,
+        // Garbage falls to `Inline`, not to `DEFAULT`: an unreadable value must
+        // not silently select an arm that needs something installed.
         None => {
             eprintln!(
                 "skill_delivery: {ENV_VAR}={raw:?} is not `inline`, `index` or `files` — \
@@ -389,6 +407,19 @@ mod tests {
         assert_eq!(resolve(Mode::Inline, true), Mode::Inline);
     }
 
+    /// The deployment default is a measured decision; changing it should fail
+    /// a test so it is made on purpose, with the numbers in front of you.
+    #[test]
+    fn the_default_arm_is_files_and_garbage_still_falls_to_inline() {
+        assert_eq!(DEFAULT, Mode::Files);
+        assert_eq!(requested_for(&serde_json::json!({})), requested());
+        assert_eq!(
+            requested_for(&serde_json::json!({ "skill_delivery": "sideways" })),
+            requested(),
+            "an unreadable per-mission value defers to the deployment, as before"
+        );
+    }
+
     #[test]
     fn the_files_arm_parses_resolves_and_reads_back() {
         assert_eq!(parse("files"), Some(Mode::Files));